This invention relates to a method of leaching zinc concentrate under atmospheric pressure.
Conventional techniques for directly leaching zinc sulfide under atmospheric pressure include the method disclosed in Japanese Patent No. 2,856,933. This known hydrometallurgical process for the production of zinc comprises, as shown in FIG. 2, steps of roasting zinc concentrate and neutral leaching the resultant calcine with a spent acid solution (spent electrolyte) from the electrolytic winning stage to obtain iron-rich neutral leach residue, which is further leached with a spent acid solution to dissolve iron precipitate contained therein and obtain a solution of ferric sulfate, which is used for directly leaching zinc concentrate. This solution of ferric sulfate turns to a solution of ferrous sulfate as the leaching of zinc concentrate proceeds and the ferrous sulfate thus produced is oxidized by blowing an oxygen gas into the solution. The resultant solution of mixed ferrous and ferric sulfates is transmitted to the subsequent step for precipitating iron, where zinc concentrate is added to the solution again to thereby convert ferric sulfate into ferrous sulfate. Then, the resulting solution is subjected to the treatment for removing iron by precipitation.
The reaction of converting zinc concentrate into soluble zinc sulfate with a solution of ferric sulfate is represented by the following formula (1). The reaction of oxidizing ferrous sulfate (reduced product of ferric sulfate) with an oxygen gas is represented by the following formula (2). The concentration of ferric sulfate in the reactions of formulas (1) and (2) can be enhanced up to 10 g/liter at most.
Fe2(SO4)3+ZnS=ZnSO4+2FeSO4+Sxe2x80x83xe2x80x83(1)
xe2x80x832FeSO4+H2SO4+0.5O2=Fe2(SO4)3+H2Oxe2x80x83xe2x80x83(2)
The method disclosed in Japanese Patent No. 2,856,933 makes it possible to recover zinc directly from zinc concentrate by dissolving zinc in the form of a soluble salt, wherein the amount of zinc which can be dissolved is proportional to the amount of iron contained in the calcine (oxidized ore) produced by oxidizing roasting zinc concentrate. Therefore, this method enables to increase the output of zinc. This method is superior to the conventional hydrometallurgical process for producing zinc, because it enables to increase the output of electrolytic zinc without reinforcing the currently available production unit inclusive of a roasting furnace and an apparatus for the production of sulfuric acid. However, this method has drawbacks in that the concentration of ferric ions is limited to a low level for the reason previously mentioned referring to the formulas (1) and (2). Therefore, the amount of zinc concentrate which can be added to balance the reaction is limited. Thus, it cannot increase the output of zinc to a maximum. Hence, the object of the present invention is to solve the above mentioned problems fundamentally and increase sharply the amount of zinc concentrate which can be added so as to make it possible to leach much more amount of zinc from zinc concentrate than the prior art, and to provide a method capable of enhancing the output of zinc without strengthening the capacities of currently available roasting furnace and sulfuric acid production apparatus.
In order to attain the above mentioned object the present inventors have made the keen investigation and as a result they have enabled to develop the present invention. That is, they have noticed that in the method disclosed in Japanese Patent No. 2,856,933 the amount of ferric ions necessary for leaching zinc concentrate is inevitably restricted, thereby restricting the amount of zinc concentrate which can be added to the pulp in the leaching stage. Under these circumstances, the present inventors have investigated to find out an appropriate way for increasing the amount of ferric ions contained in the pulp for leaching zinc concentrate.